Recently, various device structures, such as, FERAM (ferro-electric RAM), MRAM (magnetic RAM) and OUM (ovonic unified memory) have been proposed as the next-generation nonvolatile random access memories (NVRAM) capable of high speed operation so as to replace the flash memories, and they have been examined from the standpoint of improved functions, higher reliability, reduced cost, process matching, etc.
However, ideal general-purpose memories, so-called universal memories have been required to be such that they are capable of high speed access as in the case of the SRAM, capable of high degree of integration as in the case of the DRAM, having nonvolatility and low in power consumption as in the case of the flash memories, and previously mentioned various memories have been unable to meet these demands.
The characteristic features of the previously mentioned memories are as follows.    (1) FeRAM—This memory utilizes a spontaneous polarization reversing phenomenon of oxide ferroelectric materials and it features a low power consumption and high speed operation. Although it has been put in practical use, it is inferior from the standpoint of reliability, reduction in cell area, high cost and destructive reading.    (2) MRAM—This memory is one utilizing a GMR (giant magneto-resistance) effect. The ferro-magnetic tunnel effect device has a structure such that two ferro-magnetic material layers of Fe, Co, Ni or the like are held between very thin insulating layers (tunnel barrier layers) of Al2O3 or the like, and the direction of magnetization (spin) of the ferro-magnetic material layers is changed to control the magnitude of the tunnel current flowing through the insulating layers and thereby to develop a memory effect. There are problems that power consumption is high at magnetization reversal during the writing and that the desired sliming is difficult.    (3) OUM—The memory is one based on the thermal phase transformation of a chalcogenite material. While this is superior in terms of low cost and process matching, there are still problems in terms of sliming and high speed operation due to its thermal operation.
In contrast to these existing memories, Shangquing liu, Alex Ignatiev, etc., of Houston university have invented a resistance random access memory (hereinafter referred to as a [RRAM]) which is closer to the concept of universal memory. This RRAM is a resistance random access memory device utilizing an electrical-pulse-induced-resistance effect (hereinafter referred to as an EPIR effect) which has been discovered newly in materials having a colossal magneto-resistance: CMR) effect (refer to patent literature 1, non-patent literature 1 and non-patent literature 2). Note that in the discussion to follow, those materials having a perovskite type structure showing an electrical resistivity variation ranging to as high as a number of three figures due to the application of a field which is characterized by the colossal magneto-resistance effect, that is, a field-induced anti-ferromagnetic insulator to ferromagnetic metal transition phenomenon are defined as CMR materials. Also, any device utilizing the EPIR effect is defined as an EPIR device.
The CMR material is typically represented by a Mn type oxide material having the perovskite type structure and it exhibits a magnetic field dependent resistivity variation ranging as large as a number of three figures and relating to the field-induced anti-ferromagnetic insulator to ferromagnetic metal transition phenomenon as mentioned previously. However, the material singly requires the application of a considerably large magnetic field (of an order of several teslas) in order to obtain a high magneto-resistance effect. In addition, there are problems that as in the case of the previously mentioned MRAM, a high degree of boundary control is required even in the case of a device having a magnetic tunnel structure capable of operating at a weak magnetic field of about 0.1 tesla utilizing the spin polarization rate of the CMR material which is as close as 100%. Note that while it has been discovered that the switching can be effected by means of an electrical field or a light in addition to a magnetic field as an external perturbation for controlling the resistance variation (patent literatures 2 and 3) with these Mn type oxide materials, all of them are operations in low temperature regions and therefore cannot be said to be practical always.
On the other hand, the CMR materials showing the EPIR effect are typically represented by those having the perovskite structure based on the network of oxygen-octahedrals centering on 3d-transition metallic elements, more specifically Pr1−xCaxMnO3 (hereinafter referred to as a ┌PCMO┘), La1−xCaxMnO3, La1−xSrxMnO3, Gd0.7Ca0.3BaCO2O5+5 and the like. It has been said that of these materials, the PCMO having the composition close to x=0.3 shows the widest range of resistance value variation.
The EPIR effect of such CMR material is quite epoch making in that a resistance variation ranging up to a number of several figures is produced without any need of magnetic field application even at room temperature. The RRAM utilizing this phenomenon requires no magnetic field at all and therefore the consumption of power is extremely low as compared with the previously mentioned MRAM. In addition, there are excellent features that the realization of slimming and high degree of integration is easy and the dynamic range of resistance variation is markedly wide as compared with the MRAM, thus making it possible to effect a multi-valued memory operation.
The basic structure of the CMR material portion in the actual memory device is extremely simple so that it is only necessary to laminate a lower electrode layer, a CMR thin film layer and an upper electrode layer in this order on the principal surface of a substrate. By controlling the polarity, voltage and pulse duration of electrical pulses applied across the upper electrode layer and the lower electrode layer, respectively, in a wide range of several tens ns to several μs, it is possible to vary the resistance of the CMR thin film layer held between the upper and lower electrode layers thereby to perform a memory operation. The resistance value of the CMR thin film layer varied by the application of these pulses is maintained over a long period of time even after the application of the pulses so that if, for example, the low resistance state represents a logical value “0” and the high resistance state represents a logical value “1”, it is possible to obtain a nonvolatile memory function. In addition, by controlling the pulse duration, applied voltage and number of pulses, it is possible to realize a multi-valued memory operation based on a stepwise variation of resistance value with a margin which is several hundred times the variation of resistance value attained in the MRAM.
Then, while the materials used for the electrodes of the above-mentioned multilayer structure include metal types, such as, Pt, Ir, Ru, Ph, Ag, Au, Al and Ta, or materials which are higher in conductivity than the CMR material, such as, oxides including YBa2Cu3O7−x, RuO2, IrO2 and SrRuO3 and nitride type compounds, such as, TaSiN, TiN, TiSiN and MoN, the lower electrode layer is at a high temperature of 400° C.˜600° C. and also exposed to an atmosphere having a high oxygen partial pressure during the formation of the CMR layer. Thus, the range of selection of materials used for the lower electrode layer is limited.
Also, when selecting the lower electrode, it is necessary to take into consideration the standpoint of lattice matching between it and the CMR material. Such CMR material which reveals an excellent EPIR effect is strained from the ideal cubic-system perovskite structure and thus the transition metal-oxygen bonding network is greatly bent giving rise to anisotropic properties. As a result, in the case of a randomly oriented CMR thin film ignoring the crystal axis orientation, the problems of uniformity of characteristics in the substrate surface and the lot-to-lot process reproducibility are actualized due to the reduction in cell area. Further, it is necessary to take into consideration the promotion of crystallization of the CMR material due to the base structure transfer caused by such lattice matching and the improvement in characteristic due to the higher crystallization.
As a result, it is considered that in order to attain the improvement in characteristic and the control of characteristic in the light of mass production, it is effective that {circle around (1)} a lower electrode material is selected which is easy in orientation control and having the desired lattice matching with the CMR material, and {circle around (2)} the orientations in the substrate normal direction of the crystal axes of CMR crystals to be epitaxially grown are made uniform.
[Patent Literature 1]                Specification of U.S. Pat. No. 6,204,139        
[Patent Literature 2]                Japanese Laid-Open Patent Publication No.10-261291        
[Patent Literature 3]                Japanese Laid-Open Patent Publication No.10-255481        
[Non-Patent Literature 1]                Applied Physics Letter, 76, 2749–2751 (2000), “Electrical-pulse-induced reversible resistance effect in magneto-resistive films,”        
[Non-Patent Literature 2]                Nikkei Microdevice, 2003, January Number, pp.72–83        